The overall goal of this proposal is to investigate new mechanisms that underlie SCD through the application of innovative technology, computer modeling and the use of new genetic models of long QT syndrome type 1and type 2 (LQT1 and LQT2). This multi-pronged approach includes the investigation of orchiectomized male LQT2 rabbits and LQT1 rabbits that underwent ablation of the AV node (LQT1-AVB). Studies at the cellular levels with myocytes derived from these rabbits will be used to investigate the role of sex hormones in modulation of the function of key proteins that regulate calcium dynamics in cardiomyocytes. Since the calcium (Ca) transients and action potential dynamics are bi-directionally coupled through Ca-induced-Ca-release and the feedback of the Ca transient on Ca-sensitive membrane current (ICa,L, INCX, and IKs), assessing the relative contributions of unstable Ca cycling and altered repolarization to triggered activity is in general difficult. Here e propose to investigate the subcellular Ca cycling dynamics first without Vm influence to characterize RyR sensitivity, and then with a clamped action potential waveform to investigate the uni-directional effect of Vm on Ca cycling dynamics (i.e. without feedback of Cai on Vm), thereby identify conditions leading to arrhythmogenic whole cell Cai oscillations, Vm dynamics without CaT influence to study EADs facilitated by reopening of ICa,L in the presence of altered repolarization, and finally bi-directionally coupled free Cai and free Vm dynamics to investigate synergistic effects of Cai oscillations and reopening of ICa,. Iteration will consist of functional electrophysiological measurements of the specific currents and their biophysical properties, and biochemical and electrophysiological studies (as well as measured subcellular Ca dynamical properties-spark frequency and Ca wave threshold), to calibrate relevant ranges of computational model parameters to be used in the exploration of arrhythmogenic state (unsafe zone) and safe zone (no spontaneous arrhythmias or SCD). Studies at the tissue and organ levels will examine the role synchronization of the triggered activity combined with bi-stable wave conduction (bi- excitability) and tissue heterogeneities mediate the R-on-T phenomenon as well as test whether short-long-short sequence increases EAD formations and tissue heterogeneities, thus self propels until the onset of TdP and weather TdP and PVT are maintained by Ica-mediated wavefronts and chaos synchronization.